has been identified in Arabidopsis[119]. It contains three EF hand motifs (Figure 2) and the bacterially
expressed protein showed a characteristic Ca^2 -shifted electrophoretic mobility pattern. High levels of
transcript are observed in flowers and roots compared with leaves and siliques in Arabidopsis[119]. The
AtCP1gene transcripts are highly inducible by NaCl treatment but not by ABA treatment, indicating the
specificity of this unique Ca^2 -binding protein in responding to various stress factors.
TheRbohA(respiratory burst oxidative homolog A) gene from Arabidopsisand rice is also a Ca^2 -
binding protein (Figure 2). The sequence features of this gene indicate its involvement in the oxidative
burst [150]. Another cDNA, PvHra32, a transcript highly expressed during the hypersensitive reaction in
bean cell cultures challenged with Pseudomonas syringae, has been shown to encode a small Ca^2 -bind-
ing protein (161 aa) [206]. The Hra32 protein contains four EF motifs and shows 51% sequence homol-
ogy with a salt-inducible, three EF hands–containing Ca^2 -binding protein, AtCP1, from Arabidopsis
[119]. Cytosolic Ca^2 levels in both these processes are inducible and a correlation has been established
between increased Ca^2 levels and these Ca^2 -binding proteins in response to salt stress.
D. Phospholipase C
A cDNA sequence, namely AtPLC1, was isolated from Arabidopsisusing a PCR-based strategy with
primers to conserved regions of animal phospholipases [82]. AtPLC1 encodes a 64-kDa protein with
characteristic features of phosphatidylinositol-specific phospholipase C (PI-PLC) activity [82]. The E.
coli–expressed PI-PLC hydrolyzes phosphatidylinositol-4,5-biphosphate to IP 3 and diacyl glycerol with
an absolute requirement for Ca^2 (1M) [82]. IP 3 has been shown to stimulate Ca^2 release from the
vacuolar store [279], and diacyl glycerol is an activator for protein kinase C activity. Hirayama et al. [82]
showed that AtPLC1gene expression is induced by stresses including dehydration, salinity, and low tem-
perature. Under these stresses, Ca^2 and PLC may work in parallel in coordinating the environmental
stress–inducible signal transduction pathway and cellular response. For example, coordinated action of
IP 3 -Ca^2 in closure of guard cells under osmotic stress has been established. Furthermore, the presence
ofAtPLCgene activity in Arabidopsisraises the possibility of the presence of protein kinase C in plants.
AnotherAtPLC2gene is shown to be constitutively expressed in Arabidopsis[280]. Three PI-PLC iso-
forms (StPLC1to-3) have been isolated from guard cell–enriched tissue of potato [281]. The expression
pattern of the StPLC1and-2genes suggests their involvement in drought stress in potato [281]. The soy-
bean plasma membrane–associated PLC is unique in that the Ca^2 -binding domain spans across the so-
called X and Y domains [282]. The existence of a small multigene PLC family has been identified in soy-
bean [282]. The soybean PLC showed phospholipase activity and complemented the lethal mutant
phenotype of yeast lacking PLC activity. Immunolocalization of PLC in the overexpressing transgenic
PLC plants suggests that its distribution is associated with plasma membrane and cytosol [282].
E. Protease
A 75-kDa cysteine class of Ca^2 -dependent protease (CDP) has been purified from Arabidopsisroot cul-
tures [283,284]. Its activity is specifically dependent on Ca^2 but not on other divalent cations such as
Mg^2 , Sr^2 , and Zn^2 . Calcium chelator EGTA inhibits the CDP activity. The concentration of Ca^2 re-
quired to activate ACDP is more than the physiological cytosolic Ca^2 levels. However, in other reported
animal CDPs it has been shown that inositol phospholipids reduced the Ca^2 required for CDP activity
[285]. In animals, the CDPs or calpains are involved in many cellular functions by controlling proteoly-
sis of other enzymes and structural proteins including protein kinases, myofibrillar proteins, cytoskeletal
proteins, transcription factors, hormone receptors, and growth factors [285–287]. However, in plants, the
role of CDPs in Ca^2 signaling pathways is not known.
IV. CALCIUM AND GENE EXPRESSION
It is becoming increasingly clear that the regulation of expression of specific genes is involved in a plant’s
ability to adapt or develop resistance to abiotic and biotic stress factors such as cold, heat, salinity, and
pathogens [3,10,11]. The role of Ca^2 in regulating gene expression, at both the transcriptional and the
translational level, has been well documented in animal cells [288–295]. Much of the gene regulation by
Ca^2 is accomplished by Ca^2 -regulated protein kinases. Transcriptional regulation of expression of spe-
712 REDDY AND REDDY